Meagnetic liquid developer and method for electrostatic images



NOV. .1964 D. L. FAUSER ETAL 3,155,531

MAGNETIC LIQUID DEVELOPER AND METHOD FOR ELECTROSTATIC IMAGES Filed Sept. 23, 1958 FIG MAGNET MAGNETIC BRUSH WITH WETTED MAGNETIC PARTICLES' AND DEVELOPER AGENT IMAGE SURFACE H6 -2 MAGNETIC FIEL LIQUID DEVELOPER CONTAINING MAGNETICALLY ATTRACTABLE DEVELOPER AGENT COUNTER ELECTRODE IF DESIRED INVENTO S DONALD L. FAUSER a BY EDWIN R. KOLB ATTORNEYS United States Patent 3,155,531 I MAGNETEQ LIQUID DEVELOPER AND METHG FOR ELEIRQ3TATI IMAGES Donald L. Fauser, Lakewood, and Edwin E. Kolb,

Grafton, Qhio, assignors to Harris-intertype (Jorporation, (Ileveland, ()hio, a corporation of Delaware Filed Sept. 23, 1958, Ser. No. 762,699 19 Claims. (Cl. 1l737) This invention relates to electrophotography and, more particularly, to improved systems for developing and fixing latent electrostatic images using new development techniques and compositions including components having magnetic properties and characteristics.

'Electrophotographic or electrostatic reproduction processes of the character to which this invention relates include processes such as are disclosed in the copending application of Dolar N. Adams and Donald L. Fauser, Serial No. 640,353, filed February 15, 1957, and comprise generally the steps of impressing an electrostatic charge on a plate or paper or other image carrier base or sheet having an electrophotographic or electrophotosensitive surface, exposing the charged surface to a light image whereby portions of the charged surface corresponding to the light image are discharged to form an electrostatic image or pattern, and rendering the electro static pattern visible or developed by applying thereto a developing agent having electrostatic charged characteristics such that it will be attracted to the charged areas and not to the discharged areas of the exposed surface, or vice versa, and then fixing the developing agent more or less permanently in place of electrostatically deposited to form the image.

As noted, also, this invention is particularly concerned with the development steps of such electrophotographic processes, including the various types of liquid develop ing systems and techniques of the character disclosed in the copending application of Donald L. Fauser and Edwin R. Kolb, Serial No. 762,756, executed and filed of even dates herewith, it having been discovered that certain substantial advantages, enhanced quality and utility, and greater ease and control of both the development and the finished product are obtained if finely divided charged particles or droplets of developer agent to be electrostatically attracted to the charge image are applied or presented to the image dispersed in a substantially non-conductive liquid medium or vehicle, rather than when applied as dispersed in a gaseous medium or with a mechanical carrier.

In the following description and claims such expressions as developed, developer, development, etc., should be understood as not necessarily being restricted merely to the rendering of a latent or electrostatic charge image visible to the eye, as such terms are generally understood in discussions of, for example, silver halide photography. Such terms are here to be understood as including the rendering useful for its intended purpose the image areas on a charged electrophotosensitive plate, paper, or other image carrier, regardless of whether the ultimate desired utility is merely to have the image visible, as with a photographic reproduction, or to produce in charged areas a particular surface characteristic, such as the characteristic of being oleophilic in the preparation of a lithographic printing plate, or to produce a textured design surface, whether or not it may also have visual contrast with non-image areas, and the like.

Similarly, as will be understood, the particular electrostatic or electrical characteristics of a developer agent required therein to provide the desired electrostatic attraction or deposition thereof on the electrostatic image may be (and usually are) quite independent of the capacity of the particular developer material to provide the desired visible or other physiochemical surface characteristic in the finished product, and the actual developing agent may have to be supplemented, as with dyes or pigments or other constituents, to produce the ultimately desired results. In liquid developing systems, of the character to which this invention relates, where the developer agent is dispersed in finely divided form in a non-polar organic liquid, as disclosed in said Fauser and Kolb copending application, and as the complexity of the dispersed phase increases, certain complications may arise in the utilization of such developing materials. Considering the situation where, as noted in said copending application, development of the electrostatic image is accomplished by presenting to the image a liquid medium comprising electrostatically charged finely divided particles or droplets dispersed colloidally in a non-polar organic liquid for the deposition of the particles on to selected areas of the electrostatic image according to and as induced by the electrostatic attraction on the image areas for the particles, a number of considerations are important. Particularly with continuous tone images (such as the reproductions of photographs, etc.), where substantially little visual distinction or definition is desired as between wholly white background areas and, for example, very light gray mid-tone areas, the electrostatic charged gradient or differential as between the background areas (where no deposition of developer agent is desired) and adjacent light gray areas (where but little deposition of developer agent is desired) may be quite small.

When a positively charged developer agent, either dry or dispersed in a non-conductive vehicle, is flowed or brushed over such an electrostatic image, some deposition or precipitation of the developer particles or droplets may occur, perhaps from purely gravitational reasons, in background areas where no deposition or developer is desired. Similarly, perhaps because of the electric field concentrations at the edge portions of slightly charged image areas adjacent wholly discharged areas, some unwanted concentration of developer particles may occur at such edge areas. Also, the passage of liquid developer, including a high concentration of charged developer particles over large background or discharged areas of the image, particularly when an electric field is being superimposed on the charged image as by a counter-electrode (as disclosed in said Fauser and Kolb application) for the purpose of controlling deposition of developer particles, an attractive charge may actually be induced onto otherwise discharged surfaces of the image by the passage thereover of the charged particles in the developer so that an actual electrostatic attraction and deposition or precipitation of developer particles will occur, even in background areas which were completely discharged during exposure of the image, as a result of such later induced charges by the charged developer itself.

Any of such situations, as will be understood, may produce a disadvantageous or undesired accumulation or deposition of developer particles in background or discharged areas of the electrostatic image to an extent which may impair the final clarity, contrast, definition, etc., desired in the developed image.

According to this invention, however, development system and materials of the character described are provided including in the developing composition a magnetic component, so that the developer agent is responsive to both magnetic and electrostatic fields and/or influences, and conducting the developing steps under the influence of a magnetic field whereby the developer particles are magnetically attracted away from the charge image (but by a force or amount which is less than the I) electrostatic attraction of the charged areas of the image but more than the gravitational or developer-induced electrostatic attraction of developer particles to discharged or background or non-image areas of the electrophotographic plate or film being developed) for the production of developed electrostatic images having enhanced freedom of unwanted deposition or precipitation or attraction of developer materials on to the background or non-image areas thereof. As a further feature of this invention, such developer systems are provided with or without the superimposition on the system of the efiect of a counter-electrode arrangement and, since the magnetic properties of the system considered here are believed to be virtually independent of the electrical or electrostatic characteristics of the system, the advantages of a superimposed magnetic field are achieved without necessary alteration of the electric, electrostatic, visual, or other physio-chemical characteristics of the developer systems to which this invention relates. The added control of a magnetic field on the development may be, if

desired, additive (either in the attraction or repulsion of the developer, depending on the desired development thereof) to the electrostatic influences and characteristics thereof.

One object of this invention is to provide, in the development systems of the character described for developing electrostatic images, a developer composition including a magnetic component and a magnetic field acting thereon whereby the developer composition is magnetically attracted away from the surface of the charged image being developed in background or non-image areas thereof where a deposition of the developer is not desired, but is not magnetically attracted away from charged image areas where electrostatic attraction for the developer exists in sutficient strength to overcome the influences of magnetic forces.

Another object of this invention is toprovide developing compositions of the character described including a magnetic pigment or component adapted for magnetic attraction in a magnetic field away from the electrostatic image surface being developed for enhanced freedom in background areas of the image from undesired deposition of developer.

A further object of this invention is to provide, in developer systems of the character described for the development of an electrostatic image in electrophotography, a developer composition including components which are electrostatically attracted toward the charged image surface for precipitation or deposition thereon and a magnetic component adapted, under the influence of a superimposed magnetic field, to attract the developer away from uncharged areas of the electrostatic image.

Still another object of this invention is to provide, in developing systems of the character described for electrophotography, methods and conditions whereby the developer material includes a magnetic component, and a magnetic field is superimposed on the system whereby the developer material is magnetically attracted away from background or non-image areas of the surface being developed with a force sutficient to avert unwanted deposition of developer material on the background areas, but which force is not sufiicient to exceed the electrostatic attraction of developer material to charged image areas of the surface being developed.

Still another object of this invention is to provide, in electrophotographic processes of the character described, liquid development systems utilizing a finely divided charged developer material uniformly colloidally dispersed in a non-conducting organic liquid, and including a magnetic component, with magnetic conditions and c011- siderations arranged, substantially independent of the electrical or electrostatic characteristics of the system, to result in greater control of the selective deposition or electric precipitation of the developer material on to charged areas of the image being developed and enhanced freedon of unwanted gravitational or induced electrical deposition of the developer material on to background areas of the image being developed.

Other objects and advantages will be apparent from the following description and the appended claims.

FIG. 1 is a schematic view of the developing system in accordance with the present invention; and

FIG. 2 is a schematic view of a modified developing system in accordance with the present invention.

It will be understood that, in electrostatic or electrophotographic reproducing techniques of the character to which this invention relates, an electrophotosensitive film is provided on the surface of a sheet of paper or plate or other image carrier base on which a final image is to be produced. This film is capable of retaining an electrostatic charge, and may include an electrically insulating film-forming and binder component through which is dispersed a photoconductive or photosensitive component adapted to effect lowering or removal of an electrostatic charge selectively in those areas of the film which are exposed to light.

Considering as illustrative of such as elcctrophotographic process, the use of a sensitized piece of paper, an electrophotosensitive film is applied to the surface of the paper as more particularly pointed out in the above mentioned copending application of Adams and Fauser. This film, including an insulating or dielectric film forming component and photoconductive component described therein, is given a uniform negative electrostatic charge in a darkroom or otherwise in the absence of light. The charged sheet is then exposed to a light image, as for example, by projecting an image thereon through the photographic transparency. Vfhere the light strikes the film on the sheet, the original negative photostatic charge is lowered or removed through the action of the photoconductive component in the film. This leaves an electrostatic latent image or charge image on the sheet corresponding to the light image projected thereon, but such electrostatic image is, of course, invisible, and the surface characteristics of the exposed areas are, except for the electrostatic charge thereon, indistinguishable from unexposed areas of the surface.

As illustrative of one embodiment of this invention, such electrostatic charged image is, then, to be developed to render it visible or otherwise useful for the intended purpose by the application thereto of finely divided charged developer agent particles or droplets colloidally suspended in an essentially nonconducting organic liquid vehicle therefor, and thereafter, upon evaporating or otherwise eliminating the vehicle, the elcctrostatically attracted developer particles or droplets are substantially permanently fixed in place on the image surface as by fusing, forming a binding film, etc.

As noted in said copending Fauser and Kolb application, a variety of materials are provided for achieving satisfactory results with the liquid development systems and techniques of the character to which this invention relates. Thus, a non-polar, essentially non-conducting, organic liquid, such as Varnolene, gasoline, carbon tetrachloride, kerosene, etc., having a very high resistivity of the order of It) to 10 ohm centimeters, is preferred to provided the insulating medium or vehicle or carrier for the finely divided and dispersed developer agent particles or droplets. The developer agent itself is, in that application, selected to be a pigment or resin material which, when finely divided and dispersed (as a pigment or organosol) in the non-polar vehicle, is capable of taking an electrostatic charge of a desired polarity and of being dispersible, but not soluble, in the vehicle.

Satisfactory results according to this invention, however, are achieved if the developer agent pigment dispersed in the non-polar liquid vehicle is selected to be a magnetic pigment, in addition to whatever visual or electrical properties may be desired, and/or if a magnetic material is dispersed in or otherwise included in a dispersed phase of developer agent to be colloidally suspended in the non-polar vehicle and including the situation where a magnetic pigment is coated with a resin or material insoluble in the non-polar liquid vehicle, etc., so that the developer agent particles or droplets dispersed in the non-polar vehicle exhibit magnetic properties susceptible to the influence of a magnetic field superimposed on the system and oriented, with respect thereto, to give a force tending to attract the developer particles magnetically away from the image surface being developed except as the electrostatic attraction for the developer particles for the image surface exceeds the superimposed magnetic attraction or force in the opposite direction.

As illustrative of one embodiment of this invention, a metal image carrier or plate of low magnetic permeability and having on the surface thereof an electrophotographic film including a matrix of zinc oxide photoconductive particles dispersed through a film of polyacrylic acid resin was exposed through a photographic transparency to produce an electrostatic latent image thereon. This image was developed or rendered visible by the application of a suspension of magnetic iron oxide in a solution of polyisopropenyl acetate resin dissolved in isopropenyl acetate and applied to the surface of the plate immersed in a medium of Varnolene. The developer agent was electrostatically attracted to the areas of the image surface which had not been exposed to light, and such developing agent, being initially displaceable, was rendered more or less permanent by heating the plate for about five minutes at 180 C. to eliminate both the Varnolene and the isopropenyl acetate solvent and to fix or otherwise render the polyisopropenyl acetate resin with the magnetic iron oxide pigment therein permanently adhered to the plate surface in the selective image areas thereof.

Furthermore, this developing material or composition was applied to the plate while the plate was immersed in Varnolene by the use of what may be called a magnetic brush. This device is formed by immersing one end of a straight bar magnet into iron filings, resulting, of course, in a cluster of filings at the end of the magnet which may be treated and considered here as somewhat in the nature of bristles of a paint brush. Such a magnetic brush, then, coated with the developing composition, droplets of which cling thereto due to the presence of the magnetic iron oxide therein, is applied to the image and the developing composition is painted on to the surface of the plate while immersed in Varnolene, with the result that both the dark colored magnetic iron oxide and the polyisopropenyl acetate binder are transferred from the brush to the selected areas of the plate according to the light exposure thereof and the charged areas of the plate, wherein the electrostatic attraction of the image charge for the resin droplets exceeds the magnetic attraction of the brush for the magnetic iron oxide pigment therein. Such electrostatic attraction of the resin droplets from the brush to the image areas is, probably, aided by the fact that, since the magnetic brush is all of an electrically conducting composition, it acts with respect to the charged image surface as a so-called counter-electrode, as disclosed in the aforementioned copending application, which intensifies the uniform electrostatic attraction to the image by providing an electric field thereabove to which the developer droplets are introduced.

In the preparation of such a developing composition, the magnetic iron oxide (a convenient source of which is a commercial product called Plast Iron) is stirred in an isopropenyl acetate solution of polyisopropenyl acetate resin to form a desired suspension therein. The iron oxide, which is, essentially, a coating on the iron particles, can be readily observed to float off the Plast Iron as it is stirred into the solution. The solids of the resulting admixture (i.e., iron filings from which the iron oxide coating has been removed and suspended in the solution) are removed from the mixture by submerging the end of the bar magnet into the vessel containing the mixture, thus forming the brush. As the brush is withdrawn through the upper suspension of magnetic iron oxide, the iron filing bristles thereof become coated with magnetic oxide particles suspended in the resin solution, forming the magnetic developing brush. The brush, thus formed, is rinsed a number of times in clean Varnolene to remove excess resin and solvent, and is considered ready for use as a developing medium when the Varnolene rinse is no longer clouded by emulsified polyisopropenyl acetate.

ince the magnetic iron oxide alone may not have the intensity of blackness desired for the final image, it may be desired to incorporate into the developing composition a dye or similar material. Such a dye is selected to be insoluble in the non-polar Varnolene but soluble or dispersible in the resin solvent of the dispersed phase, in order to be transferred with, but not from, the resin droplets. Actually, selecting dyes of different colors permits of a Wider range of developing possibilities than if the entire pigmentation or coloration were dependent upon pigments or resins alone which exhibit the desired magnetic or electrostatic properties.

It may be noted, generally, that the Wide range of visible colored resins may be selected to form, in a non-polar liquid vehicle, an organosol of discrete and finely divided droplets or particles adapted to have the desired selective electrostatic attraction to (or repulsion from) charged or discharged areas of the latent electrostatic image to be developed. Unless such systems are inherently magnetic or include in the dispersed phase a magnetic component, however, the added advantage of use of the aforementioned magnetic field or a magnetic brush may not be available. Also, as noted, the magnetic brush, aside from its magnetic field effect on the magnetic pigment, provides a counter-electrode effect with regard to the electric attraction and deposition of the developer material. Although such counter-electrode effect can be otherwise attained (as, for example, indicated in the aforementioned copending application), certain operational advantages and conveniences (and including, among others, that of visibility of the image during the development thereof as opposed to obscuring the progress of development by a covering plate electrode) are considenably enhanced by the use of the magnetic brush above described, and particularly with magnetic pigments, to provide both a countor-electrode effect and the magnetic field background cleaning effect described provided the developer particles are magnetic or include a magnetic component.

As will be understood, a number of magnetic pigment or pigment components are available for use in connection with this invention in addition to the magnetic iron oxide mentioned abovee.g., finely divided iron, nickel, or cobalt and/or magnetic oxides thereof, or other ferroma netic materials or ferrimagnetic materials such as CuFe O etc.and such pigments can be used by dispersing the pigment alone in a non-polar vehicle, dispersing the pigment in a polar solvent solution of a resin which is in turn dispersed in the non-polar vehicle, or coating the pigment with a heat-fusible or other binder type resin insoluble in a non-polar vehicle and dispersing the thus coated pigment.

As noted, the background cleaning effect, etc., attributable to or achievable with the utilization of magnetic components in a developer material in a magnetic field and the effect on the electrostatic deposition of the developer attributable to a counter-electrode are substantially independent of each other and notwithstanding the fact that a magnetic brush, with magnetic developers, may simultaneously achieve both counter-electrode and magnetic field results. Accordingly, it may be desired to utilize the background cleaning, etc., effect of a magnetic field with a magnetic pigment without desiring also to use the counter-electrode effect which is achieved by, for example, a magnetic brush. Thus, a printing or developing effect which would emphasize the concentration of developer materials on edge portions of large image areas might be desired in some applications, while, at the same time desiring the background cleaning effect of a magnetic field. Such a situation is also in the contemplation of this invention since satisfactory results are achieved by imposing upon the developer system a magnetic field produced by a source which is too far removed from the charged surface-e.g., a large magnetic coil or other source of field to provide the effect of a counter-electrode in the electric system with regard to the electrostatic charges on the surface being developed. Similarly, if a plate type counter-electrode apparatus is utilized, the added advantage of utilizing magnetic pigments in a magnetic field may also be desired, and a separate magnetic field may satisfactorily be imposed upon the system from an outside source without regard to the fact that a plate type counter-electrode may be substantially covering the visible image area of the charged surface.

As further illustrative of the operation and enhanced results achievable by liquid developer systems embodying and for practicing this invention, and utilizing the magnetic properties and characteristics and advantagesavaib able thereby, the following specific examples are noted among those with which satisfactory results in the development of electrophotographic images according to this invention are obtained:

Example I illustrative of one magnetic liquid developer agent may be considered a liquid developer prepared from a black iron oxide pigment dispersed in a non-polar liquid vehicle such as Sohio Oil Company Varnolene 3039. This dispersion is obtained by the interjection of commercially available Plast Iron into the non-polar vehicle. After stirring or other agitation, in Varnolene, the iron filings are observed to sink to the bottom, while charged iron oxide particles float off on the surface of the dispersion, which can then be utilized, after decantation or other separation as a developing agent. Preferably, in accordance with this illustrative example, a bar magnet is introduced into the container for the magnetic attraction thereto the iron filings-to form the bristles of the magnetic brush-and, as noted, as this magnetic brush is withdrawn through the floated-off magnetic-oxide the latter pigment is attracted thereto. The exposed electrostatic charge image, being immersed in a tray or other source of nonpolar vehicle such as Varnolene, is then brushed with the magnetic brush including on the bristles" thereof the magnetic iron oxide pigment to produce consequent development of the image by the black pigment in the areas of the image surface where the electrostatic charge image force or flux exceeds the magnetic attraction of the magnetic brush for the particles away from the image surface. More or less permanent fixing may be achieved by a variety of procedures, including overcoating with a transparent film-forming binder as disclosed in said copending Fauser and Kolb application.

Example II A complete solution of Saran resin in mesityl oxide solvent (one part by weight Saran to one hundred parts by weight solvent) is prepared and a magnetic iron oxide pigment is uniformly dispersed or suspended in the resin solution. After thorough agitation and mixing, the resin solution including suspended or dispersed iron oxide is dispersed in Varnolene by stirring one part by weight of the solution into twenty parts of Varnolene. Thereafter, an electrostatic char e image on the electrophotosensitive surface is developed using this dispersion as applied to the image surface immersed in a quantity of Varnolene by a magnetic brush as in the foregoing example, and the final fixing on to the image surface of the developer material is accomplished by fusion of the pigmented resin or otherwise as indicated in said copending applic tion.

Example HI As illustrative of a magnetic developer agent embodying and for practising this invention for use in a dry or non-liquid form, a formulation may be noted including 200 parts by weight Piccolastic Resin 4358A (as produced and marketed by Pennsylvania industrial Chemical Corp, Clairton, Pennsylvania) of about 200 mesh, 30 parts by weight black magnetic iron oxide, techni al grade (as produced and marketed by Ammend Drug 6; Chemical Com ny, ltwv York, New York, which product is also appro riate for the iron oxide component of the foregoing examples), 12 parts Spirit Nigrosine SSE (as produced and marketed by National Aniline Division, Allied Chemical & Dye Corp, New York, New York) and 8 parts by wei. losol Black dye (also produced and marketed by Allied Chemical & Dye Corp). With this illustrative formulation, the mixed powders are melted and thoroughly mixed and poured out on to a brass tray for cooling and hardening. The hardened mass is then broken up and coarse ground, and finally ball-milled for some hours (perhaps of the order of 20 hours) to produce approximately about 200 mesh fine powder which talzes on a positive charge during the formation of a magnetic brush for use in the developing steps.

Example I V A further dry development formulation is illustrated by a composition including 21 parts by weight of polyvinyl butyral resin, 58 parts by weight of a rosin-modified phe nolformalde resin (such as Amberol F7i as sold by Rohm & Haas), and about 21 parts finely divided metallic nickel produced and marketed by Fisher Scientific Company, New York, New York) as a reduced powder grade of nickel, low in cobalt. The resins are melted and mixed together with the nickel powder, and the resulting composition ball-milled to the desired fineness for use as above.

Example V A further example is illustrated by ball-milling 55 parts of Epon No. 109 resin (produced and marketed by Shell Chemical Company) into a fine powder and mixing therewith, preferably in a ball mill, 45 parts by weight of finely divided Plast-lron (as produced and marketed by Plastic Metals Division of National US. Radiator Corp). The resulting mixture of powders is fused with stirring to achieve wetting of the iron with the molten resin, and the resulting mixture cooled to harden, coarse ground, and then ball-milled to the desired particle size.

As will be understood from the foregoing, the magnetic considerations and techniques disclosed herein are applicable to any of the various liquid development arrangements or systems as disclosed in said copcnding application and, with due regard, and as taught therein, to the various film characteristics and resin developer agent characteristics, are applicable with a wide variety of magnetic materialseither as the primary developing agent or as magnetic pigmentations or components thereof-end are applicable either by a magnetic brush technique or under the magnetic field otherwise superimposed upon the electric field characteristics of this liquid developer technique, with or Without a counter-electrode effect. It will also be understood that the magnetic development techniques as disclosed herein are also applicable, utilizing various magnetic materials or pigments, to electrophotographic development systems other than liquid systems, as, for example, when a magnetic pigment is brushed dry with a magnetic brush over the surface of an exposed electrophotographic image to be developed.

While the methods and compositions herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise methods and compositions, and that changes may be made therein without departing rom the scope of the invention which is defined in the appended claims.

What is claimed is:

1. A liquid developing composition for developing electrostatic charge images on an electrophotographic surface which comprises an electrically non-conductive liquid vehicle and finely divided particles of a magnetic pigment, said particles being coated with a resin binder material present in said composition as a liquid and substantially insoluble in said vehicle and said coated particles carrying an electrostatic charge for attraction and deposition on said surface in accordance with said electrostatic charge image thereon.

2. A liquid developing composition for developing electrostatic charge images on an electrophotographic surface which comprises an electrically non-conductive liquid vehicle, a solution of a resin insoluble in said vehicle dissolved in a solvent different from said vehicle, and a finely divided magnetic pigment dispersed in said resin solution, droplets of said resin solution with said magnetic pigment dispersed therein carrying an electrostatic charge for attraction to and deposition on said surface through said vehicle in accordance with said electrostatic charge image.

3. A liquid developing composition for developing electrostatic charge images on an electrophotographic surface which comprises an electrically non-conductive liquid vehicle, a solution of a resin insoluble in said vehicle and dissolved in a solvent different from said vehicle, said resin solution being dispersible in said vehicle for forming a distinct phase of liquid droplets in said vehicle, and a finely divided magnetic pigment dispersed in said resin solution droplets, said resin solution droplets with said pigment dispersed therein being capable of clinging magnetically to a magnetic brush for application to said surface and having an electrostatic charge for developing said charge image thereon when said surface is immersed in said vehicle, the outside of said droplets being wetted by said vehicle.

4. in an electrophotographic process of the character described in which an electrostatic charge image is produced on an electrophotographic surface, the developing steps which comprise immersing said charge image on said surface in a non-conductive liquid vehicle having dispersed therein a finely divided magnetic developer agent which is insoluble in said vehicle and capable of electrostatic attraction to said surface, depositing and developing agent on said surface in accordance with said charge image thereon, and maintaining in said liquid vehicle adjaoent said surface a magnetic field of a gradient magnetically attracting said developer agent away from said surface and oppositely to said electrostatic attraction of said charge image, the strength of said magnetic attraction for I said developer agent away from said surface being less than the strength of said electrostatic attraction of said developer agent toward said charge image on said surface for minimizing deposition of said developer agent on areas of said surface other than areas of said charge image to which said agent is electrically attracted.

5. In an e ectrophotographic process of the character described in which an electrostatic charge image is produced on an electrophotographic surface, the development steps which comprise immersing said surface in a non-conducting liquid vehicle, applying to said immersed surface a magnetic developer agent which is insoluble in said vehicle and capable of attraction to and depositing on said surface in accordance with said chargeimage thereon and depositing said developer agent on said charge image, said magnetic developer agent being applied from a magnetic brush and being magnetically attracted to said brush by a magnetic force which is less than the force of said electrostatic attraction of said agent to said charge image on said surface for minimizing undesired deposition of said developer agent on areas of said surface other than those to which it is electrostatically attracted in accordance with said charge image.

6. In an electrophotographic process of the character described in which an electrostatic charge image is produced on an electrophotographic surface, the development steps which comprise immersing said surface in a non-conducting liquid vehicle, forming a magnetic brush having a finely divided magnetic developer agent insoluble in said vehicle and magnetically attracted onto said brush while immersed in said liquid vehicle, said magnetic developer agent being capable of attraction to and depositing on said surface in accordance with said charge image thereon, brushing said charge image on said immersed surface with said magnetic brush, and depositing said developer agent on said surface in accordance with said charge image, said developer agent being magnetically attracted to said magnetic brush by a magnetic force which is less than the force 'of said electrostatic attraction of said agent to said charge image on said surface for minimizing undesired deposition of said developer agent on areas of said surface other than those to which it is elec trostatically attracted in accordance with said charge image.

7. In a method for the liquid development of electrophotographic charge images of the character described, the steps of forming a magnetic brush for applying a magnetic developer agent to said charge image which comprise providing a quantity of nonconducting liquid vehicle in a mixing container having fine magnetic metal particles at the bottom thereof, dispersing in said vehicle in said container a finely divided electrophotographic developing agent different from said magnetic particles and insoluble in said vehicle and having magnetic properties and an electrostatic charge, immersing a magnet in said vehicle and into said metal particles for magnetic attraction of said metal particles to the end thereof forming a magnetic brush, withdrawing said magnetic brush with said particles attracted thereon through said dispersion effecting magnetic attraction of said dispersed magnetic developer agent onto the surfaces of said particles on said magnetic brush, and bringing said magnetic brush with said developing agent magnetically attracted thereto into close proximity with a charge image for depositing said developing agent on the image as controlled by the charges thereof.

8. In a method for the liquid development of electrophotographic charge images of the character described, the steps of forming a magnetic brush for applying a magnetic developer agent to said charge image which comprise providing a quantity of nonconducting liquid vehicle in a mimng container having fine magnetic metal particles at the bottom thereof, dispersing in said vehicle in said container a finely divided electrophotographic developing agent insoluble in said vehicle and having magnetic properties and carrying an electrostatic charge, said developer agent including a finely divided magnetic pigment different from said magnetic particles dispersed in a solution of a resin insoluble in said vehicle and dissolved in a solvent different from said vehicle, immersing a magnet in said vehicle and into said metal particles for magnetic attraction of said metal particles to the end thereof forming a magnetic brush, and withdrawing said magnetic brush with said particles attracted thereon through said dispersion of resin solution and pigment effecting magnetic attraction of charged droplets of said dispersed magnetic developer agent onto the surfaces of said particles on said magnetic brush, and bringing said magnetic brush with said developing agent magnetically attracted thereto into close proximity with a charge image for deposition of said developer agent on the image as controlled by the charges thereof.

9. In a process of the character described in which an electrostatic charge image is produced on a surface, the developing steps which comprise immersing said charge image to be developed in a non-conducting liquid vehicle, applying to said immersed surface and depositing thereon a finely divided magnetic developer agent which is capable of electrostatic attraction to and depositing on said surface in accordance with said charge image thereon, and maintaining adjacent said surface a magnetic field of a gradient magnetically attracting said developer agent away from said surface and oppositely to said electrostatic attraction of said charge image, the strength of said magnetic attraction for said developer agent away from said surface being less than the strength of said electrostatic attraction of said developer agent toward said charge image on said surface for minimizing deposition of said developer agent on areas of said surface other than areas or said charge image to which said agent is electrically attracted.

10. In a process of the character described, in which an electrostatic charge image is produced on a surface, the developing steps which comprise immersing said charge image to be developed in a non-conducting liquid vehicle, crushing said surface with a finely divided magnetic developer agent including a component capable of electrostatic attraction to and depositing on said surface in accordance with said charge image thereon and a magnetic component capable of magnetic attraction away from said surface and opposite to said electrostatic attraction to said charge image, depositing said developer agent on said immersed charge image, maintaining adjacent said surface a magnetic field for applying to said magnetic component of said developer agent a magnetic force attracting said component away from said surface, and controlling the gradient of said magnetic field for attracting said magnetic component of said developer agent away from said surface notwithstanding said electrostatic attraction toward said surface for controlling the deposition of at least some of said developer agent independently of said electrostatic force of said charge image.

11. In a process of the character described in which an electrostatic charge image is produced on a surface, the developing steps which comprise immersing said charge image to be developed in a non-conducting liquid vehicle, brushing over said surface a finely divided magnetic developer agent which is capable of electrostatic repulsion from said electrostatic charge image, and depositing said developer agent on said surface in accordance with said charge image by maintaining adjacent said surface a magnetic field of a gradient magnetically attracting said developer agent toward said surface and oppositely to said electrostatic repulsion of said charge image, the strength of said magnetic attraction for said developer agent toward said surface being less than the strength of said electrostatic repulsion of said developer agent away from said charge image on said surface, for depositing said developer agent on areas of said surface other than areas of said charge image and minimizing deposition of said developer agent on areas of said charge image.

12. A liquid developing composition for developing electrostatic charge images on an electrophotographic surface in electrophotography which comprises an electrically non-conductive liquid vehicle, fine magnetic particles in said electrically non-conductive liquid vehicle, and a finely divided magnetic developer agent different from said magnetic particles dispersed in said vehicle, said agent being substantially insoluble in said vehicle and carrying an electrostatic charge for attraction to and depositing on said surface in accordance with said electrostatic charge image thereon.

13. A composition as set forth in claim 12 which also includes a resin.

14. A composition as set forth in claim 12 in which said finely divided agent is coated with a resin binder material substantially insoluble in said vehicle, said coated agent carrying an electrostatic charge for attraction and deposition on said surface in accordance with said electrostatic charge image thereon.

15. In a method for the liquid development of electrophotographic charge images of the character described, the steps which comprise forming a magnetic brush of a magnet and finely divided magnetic particles Wetted by a non-conductive liquid, bringing said magnetic brush into contact with a dispersion of non-conductive vehicle and finely divided electrophotographic developer agent different from said magnetic particles, said developing agent being insoluble in said vehicle and having magnetic properties for magnetic attraction of said developer agent to said magnetic brush, and bringing said magnetic brush with said developing agent magnetically attracted thereto into close proximity with a charge image for deposition of said developer agent on the image as controlled by the charges thereof.

16. The method as set forth in claim 15 in which said developer agent is dispersed in a resin present in said vehicle as a liquid and insoluble therein.

17. In a method for the liquid development of electrophotographic charge images of the character described, the steps which comprise bringing a magnet into contact with magnetic particles dispersed in a non-conductive liquid vehicle for magnetic attraction of said particles to the end thereof forming a magnetic brush, bringing said magnetic brush into contact with a dispersion of electrophotographic developer agent in a non-conducting liquid, said developer agent having electrostatic and magnetic properties and being a material different from said magnetic particles, and bringing said magnetic brush with said developing agent magnetically attracted thereto in close proximity with a charge image for deposition of said developing agent on the image as controlled by the charges thereof.

18. A liquid developer composition for developing an electrostatic image on an electrically insulating surface by electrostatic deposition of developer agent particles on said surface in accordance with said electrostatic image from an electrically insulating vehicle in which said developer agent particles are dispersed, which comprises an electrically insulating liquid developer vehicle, a finely divided developer agent dispersed in said vehicle, said agent being substantially insoluble in said vehicle and capable of deposition on said surface as controlled by the electrostatic image thereon, said developer agent including finely divided magnetically attractable particles, and a liquid resin present in said vehicle for deposition on said surface to effectuate substantially permanent adhesion of said deposited developer agent on said surface upon drying, and said liquid vehicle, developer agent and liquid resin being present in said composition in proportions enabling electrostatic deposition of said developer agent particles on selected portions of said surface by the electrostatic image with substantially no deposition on the other portions of said surface.

19. In a process of the character described in which an electrostatic charge image is produced on a surface, the developing steps which comprise applying to said surface a liquid developing composition comprising a nonconducting liquid vehicle and a finely divided magnetic developer agent dispersed therein which is capable of electrostatic repulsion from said electrostatic charge image, and maintaining adjacent said surface a magnetic field of a gradient magnetically attracting said developer agent toward said surface and oppositely to said electrostatic repulsion of said charge image, the strength of said magnetic attraction for said developer agent toward said surface being less than the strength of said electrostatic repulsion of said developer agent away from said charge image on said surface, for depositing said developer agent on areas of said surface other than those areas which electrostatically repel said developer agent.

References Cited in the file of this patent UNITED STATES PATENTS 786,946 Bennett Apr. 11, 1905 2,267,999 Switzer Dec. 30, 1941 2,618,551 Walkup Nov. 18, 1952 2,751,352 Bondi June 18, 1956 2,841,461 Gleason July 1, 1958 2,846,333 Wilson Aug. 5, 1958 2,851,988 Renter Sept. 16, 1958 (Other references on following page) UNITED STATES PATENTS 3,010,842 Ricker NOV. 28, 1961 2,357,290 tt Oct 21, 1953 3,052,564 Kulesla p 4, 1962 2,880,696 Clark et a1. Apr. 7, 1959 FOREIGN PATENTS 2,390,968 Giaimo June 15, 1959 1 2,891,911 Mayer et a1 June 23, 1959 5 Fmce 1955 2,898,279 Metcalfe et a1. Aug. 4, 1959 OTHER REFERE CE 2,899,335 stl'aughan Aug-11,1959 Journal of Scientific Instruments, v01. 32, February 2,907,674 Metcalfe et a1. Oct. 6, 1959 1955 and 75 2,913,353 y 17, 1959 Atkinson at 211.: Ferrography, Journal of the Frank- 2914,480 Hagopmn Nov-24,1959 10 1111, vol. 252, N0. 5, November 1951, pp. 373-381.

2,914,996 Whitham Dec. 1, 1959 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 155,,531 November 3 1964 Donald L., Fauser et, ale

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below Column 2, line 37 for or" read of line 64 for "system" read systems column 3) line 8 strike out "the"; column 6 line 4L3 for "considenably" read considerably column 8 lines 30 and 31 for phenol formalde" read phenolformaldehyde column 9 line 4 .4 for "and" read said line 64 after "thereon" insert a comma; column 11 line 15 for "crushing" read brushing Signed and sealed this 4th day of May 1965,

;EAL)

:est:

NEST W. SWIDER EDWARD J. BRENNER csting Officer Commissioner of Patents 

4. IN AN ELECTROPHOTOGRAPHIC PROCESS OF THE CHARACTER DESCRIBED IN WHICH AN ELECTROSTATIC CHARGE IMAGE IS PRODUCED ON AN ELECTROPHOTOGRAPHIC SURFACE, THE DEVELOPING STEPS WHICH COMPRISE IMMERSING SAID CHARGE IMAGE ON SAID SURFACE IN A NON-CONDUCTIVE LIQUID VEHICLE HAVING DISPERSED THEREIN A FINELY DIVIDED MAGNETIC DEVELOPER AGENT WHICH IS INSOLUBLE IN SAID VEHICLE AND CAPABLE OF ELECTROSTATIC ATTRACTION TO SAID SURFACE, DEPOSITING AND DEVELOPING AGENT ON SAID SURFACE IN ACCORDANCE WITH SAID CHARGE IMAGE THEREON, AND MAINTAINING IN SAID LIQUID VEHICLE ADJACENT SAID SURFACE A MAGNETIC FIELD OF A GRADIENT MAGNETICALLY ATTRACTING SAID DEVELOPER AGENT AWAY FROM SAID SURFACE AND OPPOSITELY TO SAID ELECTROSTATIC ATTRACTION OF SAID CHARGE IMAGE, THE STRENGTH OF SAID MAGNETIC ATTRACTION FOR SAID DEVELOPER AGENT AWAY FROM SAID SURFACE BEING LESS THAN THE STRENGTH OF SAID ELECTROSTATIC ATTRACTION OF SAID DEVELOPER AGENT TOWARD SAID CHARGE IMAGE ON SAID SURFACE FOR MINIMIZING DEPOSITION OF SAID DEVELOPER AGENT ON AREAS OF SAID SURFACE OTHER THAN AREAS OF SAID CHARGE IMAGE TO WHICH SAID AGENT IS ELECTRICALLY ATTRACTED. 